A key step in designing vehicles with a low radar signature is isolating the contributions of various vehicle configuration details to the vehicle's total radar cross section or RCS, which may be regarded as a flat, two-dimensional radar image of the vehicle. For our purposes, configuration details would include individual components of the vehicle such as hatch covers or gun tubes or antennae. Such details would also include vehicle geometry items such as comers, edges and surface shape of hulls, bodies, cabs or turrets. Configuration details would typically further include juxtapositions of individual components with each other or with vehicle geometry items.
One method of obtaining a vehicle's RCS is by using an inverse synthetic aperture radar, or ISAR, which is similar to the synthetic aperture radar, or SAR, technique used by aircraft to view a selected target. By way of background, as illustrated in FIG. 1, a SAR system produces high-resolution images by combining radar returns from several locations along a flight path. An aircraft 10 flies along path 12, directs its radar at stationary target 14 and receives radar returns at points 16a through 16g along the flight path. By integrating the radar returns, the aircraft's relatively small radar simulates a very large, much more accurate antenna.
The basic difference between the synthetic aperture radar technique and ISAR is that, in ISAR, the target is moved in a controlled fashion while the antenna remains stationary. Particularly, as seen in FIG. 2, the target 14 is rotated on turntable 18 while being imaged by a stationary radar antenna 20 mounted on tower 22. The ISAR system combines radar returns from a wide rage of azimuths relative to the target, thus again synthesizing a large antenna.
Like a synthetic aperture radar system, an ISAR system produces a flat, two-dimensional image of a target. The mathematically simplest image formation occurs when the radar antenna has a zero elevation relative to the target and radar signals have a central line of travel 24 intersecting the target's rotational axis 26, as is the case in FIG. 3. The range R from the antenna of a given scattering center 28, or radar reflecting zone, on the target is determined conventionally from the time of flight of the echo. The rotation of the scattering center on the turntable creates a Doppler shift from which the cross range dimension C can be determined. The cross range dimension is the distance of the scattering center from the turntable's rotational axis along a line normal to line of travel 24. The range and cross range values may be regarded as vectors within an ISAR plane.
FIG. 4 shows the more general case where a radar is at a nonzero elevation angle wherein an ISAR plane 30 can tilted relative to a horizontal plane 32 and is not necessarily parallel to the rotational path 34 of a scattering center on the turntable. The angle of elevation, .phi., is now a variable in calculating the aforementioned Doppler shift experienced by the radar. In practice, a two-dimensional Fourier transform simultaneously performs the calculations necessary to determine the range and cross range values.
Although the ISAR image is generated by signals from radar 20, the point of view, or apparent position of from which the ISAR image is perceived, is not that of radar 20. Rather, the point of view lies on a line perpendicular to the ISAR plane. For example, if the radar antenna has a zero elevation relative to the target and radar signals have a central line of travel 24 intersecting the target's rotational axis 26, as is the case in FIG. 3, then the point of view lies along axis 26.